Oxygen coming in a little lower, excellent source of fuel (8)
I believe the answer is:
coalface
'source of fuel' is the definition.
(part of a coal mine)
'oxygen coming in a little lower excellent' is the wordplay.
'oxygen' becomes 'O' (atomic symbol for oxygen).
'coming in' indicates putting letters inside.
'a little lower' becomes 'calf' (I've seen this in another clue).
'excellent' becomes 'ace' (I've seen this before).
'o' inserted inside 'calf' is 'coalf'.
'coalf'+'ace'='COALFACE'
(Other definitions for coalface that I've seen before include "Where the real work takes place!" , "At which miner works" , "Where the action is - in a mine?" , "Part of mine" .)